The invention relates to a method for tubular rotary ball milling or mill with grinding instruments, comprising at least two grinding compartments separated by a partition, the mill being passed through by a current of sweeping air and working in a closed circuit manner.
The partition which is situated between two of the grinding compartments, makes it possible to control the quantity of material in the compartment situated upstream.
The invention is particularly intended for ball mills, or mills with similar grinding instruments, for cement, in a closed circuit. Generally, these mills have two compartments, namely a preparing compartment with balls with a diameter of 90 to 60 mm and a finishing compartment with balls with a diameter of 40 to 20 mm. In order to avoid dust and excessive heating of the cement during grinding, cement mills are passed through by a current of sweeping air, from upstream to downstream. The partition which separates the first and second compartment of these mills is for this reason called the intermediate partition.
In cement mills, the raw material is supplied at approximately 80% under the size of 20-25 mm, and the work of the first chamber consists of reducing it to approximately 100% under the size of 5 mm, with 95% under the size of 2.5 mm. In fact, to obtain a good grinding efficiency, it is necessary for there to be a large quantity of 20 mm balls in the second compartment. Moreover, these balls work well only insofar as the fineness criteria given above are respected at the inlet of the second chamber, and in particular as there are practically no more particles greater than 5 mm in size.
In a cement mill, the intermediate partition has several functions, it must:
retain the large balls upstream, and the finer charge of the second compartment downstream; PA1 prevent the coarse particles from leaving the first compartment; PA1 allow the fairly fine material to pass to the second compartment; and PA1 allow the sweeping air to pass.
The above functions can be carried out with single-walled or double-walled partitions. The invention relates to double-walled partitions.
Single-walled partitions are actually practically never used in modern cement grinders; since they wear out on both their faces, they have too short of a lifetime, and in addition their resistance to stresses in the axial direction of the mill is not sufficient. Moreover, with a single partition, the sweeping with air has a very small influence on the filling of the first compartment with material, and an essential characteristic of the invention--using the sweeping of air as a regulating means--could not be produced efficiently.
Patent DE-A-2,133,431 describes a typical embodiment of double-walled partitions used for separating the grinding compartments of modern cement mills. The partition includes a framework consisting of openwork segments, onto which there are bolted, upstream, grilles which allow the cement to enter the partition, and, downstream, shielding plates which make a central discharge opening. The grilles and the rear plates are subjected to strong wear only on one side, and because of this they have better durability than the grilles of single-walled partitions; when they are replaced, the framework is kept.
Slots of approximately 6 mm are provided in the grilles of the upstream wall of the intermediate partitions, the edge of the slots undergoes a small degree of working by the impacts of the balls, the particles larger than 5 mm cannot pass through these slots. If the slots are larger, in the event of disturbance of the operation of the mill, even for a short time, excessively large particles can pass into the second compartment, and they will remain trapped in the balls of 20 mm diameter which are too small to reduce large particles, and this can hamper the running of the mill for several days, with a loss in capacity which can be up to 20%.
Although it is possible, by dimensioning the slots of the upstream wall of the intermediate partition, to fulfil the first fineness criterion at the outlet of the first compartment, i.e. 100% under 5 mm in size, it is impossible to make slots which satisfy the second criterion, i.e. 95% under 2.5 mm in size. In fact, it is not possible to provide sufficiently small openings in the cast steel pieces which, for reasons of wear resistance, make up the upstream face of the partition: the sand cores used to obtain the slots when the metal is cast would not have the required mechanical strength. Even if it were possible to make such small slots, they could not be used, because the passage area would not be sufficient to let through the very high flow rates of cement and air which pass through a modern cement mill.
The work of the first compartment must consequently be such that when the material arrives at its outlet end, against the intermediate partition, it is perfectly prepared, because with the exception of the coarse particles, it will pass freely through the slots of the upstream wall.
In a large cement mill, in closed circuit, the dwell time of material in the first compartment is of the order of 2 to 3 minutes. To reduce the materials to the requisite fineness under these conditions, the dwell time is an essential element.
The dwell time depends directly on the filling of the compartment with material, if there is too little material, the dwell time in the grinding bodies is too short; if there is too much material, the dwell time is too long, and the working of the grinding bodies is too greatly damped; in both cases, the grinding in the first chamber is not sufficient, and the material is not optimally prepared therein.
The framework sectors are provided with lifters which, during the rotation of the mill, lift the material which has penetrated through the slots of the grilles to the top of the mill, from where the material falls onto a cone which diverts it towards the downstream compartment. At the center of the cone, a grille allows the air to pass and prevents the balls from passing from one compartment to the other. The lifters and the cone must be very powerful, in order to be capable of treating the highest throughputs which the mill may be called on to transport, the material which penetrates into the partition is very quickly transferred into the second compartment, the small compartment formed by the double wall of the partition contains little material and has only a small retention effect on the material contained in the first compartment.
Moreover, in a cement mill, the balls of the first compartment are relatively coarse--with a diameter of from 90 to 60 mm--to crush the material supplied to the mill. Such balls are highly permeable to the passage of the material; when it is not retained by the intermediate partition, and such is the case of partitions similar to those of Patent DE-A-2,133,431, there is generally too little material in the first chambers.
If there is too little material in the first compartment, it has been seen that the dwell time therein is too short and the material is poorly prepared for the second compartment, but this has other drawbacks:
a proportion of the grinding bodies then works without material, and hence where is a loss in efficiency;
The shieldings and the balls of the mills, which are subjected to high wear stresses, are cast from very hard alloy castings which are the most economical, and when there is too little material mixed with the balls, these casts splinter and break, which causes maintenance problems and a highly expensive loss in capacity.
Patent GB-A-1,248,251 describes a particular form of partition, which comprises an upstream face pierced with slots, except for the center, and a solid downstream wall, except for a central opening which is optionally protected by a mesh, and in the preferred embodiment, there is no lifter inside the partition, the material being discharged into the downstream compartment by overspill. With these partitions, there is most often too much material in the first compartment, which can be corrected only by irreversibly increasing the diameter of the central opening.
In view of the importance of keeping a quantity of cement in the first chamber which suits the working conditions, neither too much nor too little, the attempts have for some years been made to use intermediate partitions for controlling the quantity of material mixed with the balls, in a regulatable manner. For mechanical reasons, the various attempts made in this direction have long been ineffective. Mention may be made on this subject of U.S. Pat. No. 1,787,897, whose regulatable parts seize up rapidly.
More recently, new types of partition have been proposed with a view to reliably regulating the level of the material in the first compartments of cement mills.
The method which has proved most effective consists in using a double-walled partition and in regulating the level of material between its two walls; the level of the material in the compartment upstream of a partition actually depends on the level in the latter.
Belgian Patent BE-A-763,140 relates to a partition with rotating blades which can be actuated continuously, during the running of the mill. The rotation of the blades makes it possible to control the level in the partition and in the upstream compartment. Unfortunately, the mechanism for controlling the blades proved difficult in the environment of a grinding factory; despite various improvements, the control of the blades according to Belgian Patent BE-A-736,140 never reached the required reliability, and few industrial applications have been made of these partitions.
Belgian Patent BE-A-851,835 relates to a partition with manually controlled rotating blades. These partitions have undergone very widespread industrial developments since their inception, and the concept of a regulatable-level partition is very widespread in the cement industry. However, in order to rotate the blades, the mill must be shut down, it must be allowed to cool, the manhole of the second compartment must be opened and the mill must be entered. This takes several hours in total, and since certain cement mills change their type of production several times per day, it is not possible to consider regulating the blades for each type of product. The blades are therefore set in a compromise position for all the types of production, which does not correspond to the optimum for each type of production. Moreover, changes in the grindability of the material fed to the mill may require a different charge of material--for example, to optimize the grinding when the material is moist, it is advantageous to reduce the quantity of material in the first chamber. The partition according to Belgian Patent BE-A-851,835 is therefore an interesting solution, but because it cannot be adjusted during the running of the mill, it is a flawed solution.
Patent DE-A-3,903,256 presents another solution; for controlling the level of material in the partition, the position of a ring is adjusted, so as to adjustably enclose the passage openings, through which the material can escape towards the center from the peripheral zone fitted with lifters. The partitions according to Patent DE-A-3,903,256 have the same drawbacks as those of Patent BE-A-851,835: they cannot be regulated continuously during rotation of the mill.
In summary, existing double-walled intermediate partitions can be split into two categories:
Those which include mechanical transport means, generally consisting of a set of lifters and a cone for transferring the product from one compartment to the other. Sometimes the cone is absent and replaced by another diverting device, such as for example the inclined end plate of the blades in Patent BE-A-851,835; there are sometimes regulating means, such as the rotating blades of the same patent BE-A-851,835, which set the filling in the partition; but the substance of the principle remains that of mechanically transporting the material through the partition.
Those with a barrier effect, where a level is ensured upstream by dimensioning an overspill threshold, for example the diameter of the central opening in Patent GB-A-1,249,251. The partitions according to U.S. Pat. No. 1,787,897, already mentioned, can be likened to this; they would allow, if they could be produced without their mechanism seizing up, the slots of the upstream wall to be closed progressively from the periphery towards the center, the progressive closure of the slots producing a barrier effect with an adjustable threshold.